The production of chlorine and sodium hydroxide (chlor-alkali), about 45 millions of tons of chlorine per year, is carried out in electrolytic cells of different kinds, among which the mercury cathode electrolytic cell is of particular relevance, accounting for a production of about 12 millions of tons of chlorine per year.
In FIG. 1 a typical structure of a cell of this kind is outlined, consisting in an iron case (1) on whose bottom (2) the mercury amalgam (3) constituting the cathode flows. The anode is made by a multiplicity of electrodes shaped as a grid array (4), supported by mobile frames (5), preferably controlled by microprocessors having the purpose of regulating the interpolar gap, which may vary during the cell operation.
As 12 millions of ton chlorine/year are produced in cells at the following average operative conditions:
Current density:10kA/m2Anode/cathode voltage:4.05VFaradaic yield:96%Energy consumption:3185kWh/ton Cl2this kind of technology involves a consumption of about 38 millions of MWh/year.
In consideration of the high amount of involved energy and of the continuous increase in the cost of electricity, the cell technology has been remarkably enhanced in the course of the years, with the aim of reducing the energetic consumption, representing the most relevant item in the production costs.
Among the numerous technological innovations which contributed the most to decrease the energetic consumption, the replacement of the graphite consumable anodes with the metallic anodes has to be emphasised: the latter are typically made of titanium or other valve metal, coated with electrocatalytic material generally based on noble metals and/or oxides thereof. This type of anode, an example of which is given in U.S. Pat. No. 3,711,385, is still commercialised under the trade-mark DSA® by De Nora Elettrodi S.p.A, Italy.
It consists in a metallic structure comprising one frame and one grid array, overlapped and mutually welded or somehow fixed; the frame performs the function of mechanical support and of element of direct electric current distribution to the surface of the grid array, which is coated with an electrocatalytic film specific for the chlorine evolution reaction, and constitutes the anodic active surface.
The geometry of the grid array plays a role of great importance on the efficiency of the electrolysis process and on the energetic consumption of a cell as it influences, in a determining way, both the voltage and the faradaic yield thereof. In fact, the anode/cathode voltage of a cell, expressed as Volts, can be calculated by means of the relationship:V anode/cathode=3.15+Kf×J wherein J is the current density impressed to carry out the electrolytic process, expressed as kA/m2, and the Kf term (o “Key factor”) incorporates all the components of resistive origin. The most important factors of such resistive components, namely the ohmic drop within the anodic structure, the ohmic drop in the electrolyte due to the bubble effect, and the ohmic drop in the electrolyte due to the interpolar gap, all depend from the anodic geometry; it is one of the main objects of the invention, in particular, to minimise the two latter factors.
The bubble effect is a measure of the increase of ohmic resistance in the electrolyte due to the gas bubbles developing on the anodic surface of the grid array and interrupting the electric continuity within the electrolyte itself. In particular, the bubble effect mainly depends on the number and size of the gas bubbles that are generated upon the anodic surface of the grid array and stagnate on the immediate vicinity thereof between the anode and the cathode; it further depends on the bubble ascending velocity, and on the descending velocity of the degassed electrolyte.
In summary, the bubble effect depends from the actual current density on the anodic surface (which determines the amount of bubbles developing per unit time), from the grid array geometry (which determines the ratio between actual working surface whereupon the gas is evolved and projected surface, as well as the gas withdrawal resistance), and from the optional added devices directed to improve the fluid dynamics. In particular, it is a first object of the present invention to provide an anodic grid array geometry producing a bubble effect minimisation.
Even in the absence of bubble effect, the ohmic drop within the electrolyte is directly proportional to the interpolar gap, so that it is extremely important to bring the anodic surface as close as possible to the mercury cathode, adjusting the gap between anode and cathode surfaces in a progressive fashion. It is however necessary to maintain a certain margin of safety, to avoid the mercury touching in some points the anodic surface, causing hazardous short-circuiting phenomena. For this reason, it will be possible to maintain as lower the interpolar gap as better is the planarity of the anodic structure. It is a further object of the present invention to provide an anodic grid array geometry with enhanced planarity characteristics with respect to the prior art.
In the most recent industrial cells, operating in ideal conditions, the Kf is normally comprised between 0.065 and 0.085 V m2/kA, depending on the cell size, the type of anode and the system of interpolar gap adjustment the cell is equipped with, whereof:    ˜0.0070–0.0080 V m2/kA are attributable to the ohmic drop within the anodic structure.    ˜0.0310–0.0410 V m2/kA are attributable to the bubble effect in correspondence of the anodic surface.    ˜0.0270–0.0360 V m2/kA are attributable to the ohmic drop in the electrolyte, as a function of the interpolar gap.
In other words, about 10% of Kf is attributable to the anode structure, about 50% to the bubble effect, and the remaining 40% to the interpolar gap.
For a given cell and at given process conditions, the minimum obtainable Kf is therefore a property of the anode, to a large extent attributable to the grid array characteristics (in the order of about 90%), as it depends from the width of the region affected by the bubble effect and from the planarity of the grid array itself.
For this reason, since the introduction of the metallic anodes, the grid array has been the object of several inventions, among which are recalled for their industrial relevance:                The cited metallic anode of U.S. Pat. No. 3,711,385, which in the earliest industrial embodiments comprised a grid array made of meshes, or more commonly of a multiplicity of titanium rods of about 3 mm diameter and 4.5 mm pitch, disposed in parallel, and supported by a current distributing frame, in its turn made of rectangular titanium conductors. Although enjoying a remarkable success at the time of its introduction, this configuration presented some big limitations, due both to the bubble effect and to the shielding effect of the rods on the cathode surface, with consequent difficulties of electrolyte circulation and of gas withdrawal when operating at high current density and reduced interpolar gap. The best known industrial results with this type of anode, commonly called “Rod type anode”, operating at 10 kA/m2, are the following:        
Anode/cathode voltage:4.00VKf:0.085V m2/kAFaradaic yield:~96%Energy consumption:3146kWh/ton Cl2
With the purpose of overcoming these drawbacks, U.S. Pat. No. 4,263,107 discloses hydrodynamic baffles, mounted on the upper part of the grid array, which generate convective motions so as to reduce the bubble effect, improve the fluid dynamics and ensure an effective renewal of the electrolyte.
The shielding effect of the rods has been subsequently reduced with the introduction of the invention disclosed in U.S. Pat. No. 4,364,811 according to which a grid array made of a multiplicity of rectangular strips, about 1.5 mm thick, 5 mm high and 4.0 mm spaced, defined as blades, arranged vertically respect to the cathode, was coupled to a frame of the prior art. The best known industrial results with this type of anode, operating at 10 kA/m2, are the following:
Anode/cathode voltage:3.90VKf:0.075V m2/kAFaradaic yield:~96%Energy consumption:3067kWh/ton Cl2
Even better results were obtained by coupling the hydrodynamic means of U.S. Pat. No. 4,263,107 to a grid array made of triangular strips, with their vertex facing the mercury cathode, as disclosed in the Italian Patent n°1.194.397. This new configuration, in which said triangular strips have, as typical dimensions, 2.2 mm base, 3.7 mm height, rounded vertex of 0.5 mm diameter and pitch (intended as distance between the axis of two consecutive strips) 3.5 mm, has brought to an important reduction of the bubble effect and of the shielding effect of the rods, and to a sensible improvement of the fluid dynamics.
The best industrial results obtained with this type of anode, still commercialised by De Nora Elettrodi S.p.A under the trade-mark RUNNER®, operating at 10 kA/m2, are the following:
Anode/cathode voltage:3.80VKf:0.065m2/kAFaradaic yield:~96%Energy consumption:2988kWh/ton Cl2
An alternative solution was proposed in U.S. Pat. No. 5,589,044, which discloses a frame similar to the previous ones, coupled to a grid array made of a multiplicity of rectangular strips and specially configured with the purpose of increasing the actual surface in correspondence of the vertical sides, and of decreasing the bubble stagnation effect on the surface facing the cathode. Although the results obtained with this kind of grid array are better than those obtained with the grid array of U.S. Pat. No. 4,364,811, they still remain inferior to those obtained with the grid array of IT 1.194.397.
The above described grid array configurations of the prior art, different in terms of hydrodynamic properties, bubble effect and shielding effect on the cathode, have however two different aspects in common:                the overall anodic surface planarity is hampered by the fact that the tolerances relative to the multiplicity of elements constituting the grid array (rods, blades or strips) and to the welds needed to fix the latter to the frame add up to the tolerances related to the frame itself. For all the grid arrays of the prior art, the typical tolerances along the anodic surface range between 0.5 and 1 mm, although recurring to rather controlled and sophisticated (and thus costly) machining.        the restoration of the catalytic properties of exhausted electrodic structures (to be repeated with cycles ranging from 2 to 5 years, depending on the operative conditions of the plant) involves complex and very expensive working consisting in the removal of the exhausted coating with mechanical (sandblasting) and chemical (etching) means frequently producing mechanical distortions; in some cases therefore, additional working is needed for restoring the planarity of the grid array, before (or after) providing a novel catalytic coating. The performances of a reactivated anode are virtually never quivalent to those of an anode of new construction, both because the planarity reinstatement is never perfect, and because the removal of the exhausted coating cannot be sometimes completed, or because the material constituting the grid array itself undergoes morphological changes that are not totally reversible. Finally, it is mandatory to remove the whole electrocatalytic coating even if just part of it has been consumed, in order to recover the full operability of the active surface. This involves a remarkable and useless consumption of matter consisting in extremely expensive precious metals such as ruthenium, iridium, platinum, and so on.        